Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07D—HETEROCYCLIC COMPOUNDS
  • C07D401/00—Heterocyclic compounds containing two or more hetero rings, having nitrogen atoms as the only ring hetero atoms, at least one ring being a six-membered ring with only one nitrogen atom
  • C07D401/02—Heterocyclic compounds containing two or more hetero rings, having nitrogen atoms as the only ring hetero atoms, at least one ring being a six-membered ring with only one nitrogen atom containing two hetero rings
  • C07D401/12—Heterocyclic compounds containing two or more hetero rings, having nitrogen atoms as the only ring hetero atoms, at least one ring being a six-membered ring with only one nitrogen atom containing two hetero rings linked by a chain containing hetero atoms as chain links

Definitions

• Certain azapirones such as the compounds of Formula II, have been shown to have therapeutic potential when hydroxylated to form hydroxyazapirones of Formula I.
• Two examples of hydroxyazapirones are 6-hydroxybuspirone (R and R arel,4-butandiyl and n is 4) and 3-hydroxygepirone (R 1 and R 2 are methyl and n is 4).
• R and R 6-hydroxybuspirone
• R 1 and R 2 are methyl and n is 4
• these compounds are now believed to be biologically active and their use in treating anxiety disorders and depression has been disclosed (Mayol, R. F. U.S. patent 6,150,365, 200O; Rider, P.H. PCT appl. WO 02/16347, 2002).
• improved methods for their production would be of benefit.
• the invention disclosed below improves upon these processes by employing a. one step procedure using commercially available reagents and air.
• the process also provides direct crystallization of the product rather than chromatographic purification.
• This invention describes an improved one-pot process for hydroxylation of certain useful azapirone psychtropic agents, such as buspirone and gepirone.
• the process uses commercially available reagents and air.
• the pure product is crystallized directly from the reaction mixture, and the process is amenable to large-scale synthesis.
• the invention provides a process for the preparation of hydroxyazapirones of Formula I,
• R 1 and R 2 are independently hydrogen or C 1-6 alkyl, or where
• R 1 and R 2 taken together are -CH 2 (CH 2 ) 0- 5CH 2 - and n is an integer from 2 to 5, from azapirones of Formula II.
• a Formula II compound is dissolved in a suitable aprotic solvent to a preferred ratio of 10-20 mL/g.
• suitable solvents for enolate generation include ethereal solvents such as diethylether, 1,2-dimethoxyethane, dioxane, and 2-methyl tetrahydrofuran. Tetrahydrofuran (THF) is a preferred solvent for this reaction.
• a suitable reductant in the range of 1-5 equivalents is added to the solution. Suitable reductants are those that reduce organic hydroperoxides to alcohols.
• reductants include tri(Ci- 8 )alkylphosphites as well as other reductants such as triarylphosphites, triaryl- and trialkyl phosphines, thiourea, sodium borohydride, copper (II) chloride with iron (II) sulfate, iron (III) chloride, titanium isopropoxide, dimethyl sulfide, diethyldisulfide, sodium sulfite, sodium thiosulfate, zinc and acetic acid, and 1-propene. While the reductant may be added at any convenient stage of the process, it is preferably present when the oxygenation reaction proceeds. The solution is cooled to -40 to -100 °C, preferably to a range of -68 to -75 °C, and allowed to completely stabilize.
• an appropriate strong base mediates deprotonation and formation of an imide enolate anion (HI).
• Preferred bases suitable for this type of deprotonation include disilazanes, such as lithium bis(trimethylsilyl)amide, sodium bis(trimethylsilyl)amide, and potassium bis(trimethylsilyl)amide.
• Other strong bases which may be used include dialkylamide bases (such as lithium diisopropylamide), metal hydrides, and metal alkoxides.
• Generation of a stoichiometric amount of enolate is critical for optimizing the process— undergeneration of enolate resulted in poor conversion and recovered starting material, while overaddition of base resulted in the production of dihydroxylated side products.
• reaction monitoring in particular employing FTLR, to directly observe conversion of the starting imide to the corresponding enolate solved this issue.
• Direct observation of anion generation allowed the base to be charged until the IR signal for starting material no longer declined, indicating complete consumption of the starting material.
• Starting material was then incrementally charged until a steady IR signal of starting material was observed, indicating no excess base was present.
• This provided a solution of enolate with a slight excess of starting material (1% to 3%). Because excess starting material was easier to purge than dihydroxylated side products (the impurities that resulted when excess base was present), this was the preferred situation. Variations in the base titer, water content, and phosphite quality were automatically corrected because the phosphite was charged before the base.
• the enolate compound of Formula III was formed in situ and reacted immediately with an electrophile.
• the invention includes stable enol compounds which can later be reactivated.
• enol acetates and enolsilanes are suitable substrates for the process.
• air or oxygen was sparged into the reaction mixture, controlling the initial rate of sparging to maintain the temperature of the reaction mixture less than -40 °C. The sparging was continued until the reaction was complete as indicated by HDPLC.
• air and oxygen gas are preferred sources of molecular oxygen, other oxygen sources can be used including gaseous mixtures containing molecular oxygen, liquid oxygen, and solutions containing liquid oxygen.
• the mixture was diluted with a suitable solvent such as methyl tert-butylether (MTBE), ethyl acetate, or 2-methyl-THF, warmed to room temperature, and neutralized with 1M hydrochloric acid until the pH was 6.0 to 7.0, preferably 6.5 to 6.9. Other acids can be used and the final pH can also be adjusted with various bases including sodium phosphate.
• MTBE methyl tert-butylether
• 2-methyl-THF 2-methyl-THF
• reaction product contained recovered starting material or a 6,10- dihydroxylated side product.
• one of two crystallization procedures improved the purity. These procedures are described in the Specific Embodiments section.
• the solvent of the rich organic layer was then replaced by isopropyl alcohol and the solution was cooled to crystallize the reaction product. There is an option to seed with 0.01 to 5 % buspirone at approximately 54 to 56 °C.
• the crystalline slurry was then filtered and the wet cake was washed with isopropyl alcohol and dried to provide 6-hydroxybuspirone (220.0 g, 82%), mp 109.5 °C.
• the product can be crystallized by either of these two methods:
• 6-hydroxybuspirone (35.0 g, 90.8 mmol) was slurried with anisole (385 mL, 1 1 mL/g, 10-15 mL/g may be used). The mixture was heated to 80-100 °C and stirred to obtain a clear solution. The solution was then cooled to 75-85 °C before 6,10-dihydroxybuspirone seeds (87.5 mg, 0.25 wt%, 0-2 wt% may be used) were added. The mixture was then cooled to ambient temperature over 2-6 h and stirred overnight. The resulting slurry was filtered and the filtrate was concentrated to approximately half its initial volume. Heptane (400 mL) was then added over 1 h and the resulting slurry was stirred at ambient temperature overnight.
• 6-hydroxybuspirone (220.0 g, 547.9 mmol) was slurried with absolute ethyl alcohol or isopropyl alcohol (2.20 L, 10 mL/g, 10-20 mL/g may be used) in a 3-necked round bottom flask equipped with a mechanical stirrer. The mixture was heated from ambient temperature to form a solution (55-70 °C). The resulting solution was then cooled to form a slurry. The solid was filtered, washed, and dried to provide purified 6-hydroxybuspirone (165.0 g, 75% recovery). In some experiments, this recrystallization reduced buspirone from -3% to -1.4%.
• Buspirone 350.0 g, 0.908 mol was dissolved in THF (6.9 L) in a 10 L vessel under argon. The mixture was cooled to -70 °C using a dry ice/TPA bath. A THF solution of NaHMDS (0.908 mol, 1.00 equiv [0.762 mol titrated to be 0.953 M and 0.146 mol titrated to be 0.91 M]) was added over 12 min while maintaining the temperature below - 40 °C. Triethylphosphite (3.18 mol, 3.5 equiv) was added in one portion over approximately one min. The solution was stirred at -60 °C for approximately 45 min. The solution was then cooled to -70 °C.
• Oxygen (ultra high purity [UHP]) was bubbled into the reaction mixture using a gas dispersion tube. (Note: the bubbler and a Nitrogen inlet were configured so that nitrogen passed through the vessel during the entire reaction). An exotherm of approximately 5 °C was observed and the rate of oxygen sparging was controlled so as to maintain the temperature below -64.6 °C.
• the reaction was monitored using HPLC by taking aliquots of the reaction mixture and quenching into the organic mobile phase. When the AP of the starting material no longer declined (AP approximately 2), the reaction was quenched -with HC1 (6 M, 0.5 L, the pH was measured to be approximately 3 at approximately —10 °C) and allowed to warm to room temperature overnight.
• the pH was adjusted to approximately 2.0 by the addition of NaOH (2 JV, 40 mL) and the solution was observed to be cloudy and somewhat heterogeneous.
• HPLC analysis indicated 6-hydroxybuspirone (92.6 AP) and buspirone (1.27 AP).
• the mixture was transferred to a 22 L, jacketed 3-neck flask equipped with mechanical stirrer, gas adapter, reflux condenser, and thermocouple. Water was added (650 mL) and the mixture was heated. At approximately 35 °C, the mixture became homogeneous. (Note: alternatively one can add 2.5 M HC1 instead of 6 M HC1 and water to adjust the pH to 2.0). The mixture was then warmed to approximately 58 °C for a total of approximately 30 h and held at ambient temperature for approximately 124 h.
• the solution was then neutralized to a pH of 6.84 by the slow addition (25 min) of a NaOH/saturated brine solution (3.5 N, 1.0 L [700 mL of 10.0 NNaOH and saturated brine added until the volume reached 2.0 L]).
• MTBE 650 mL
• saturated brine 500 mL
• the aqueous layer (2900 mL) was removed and saved for analysis and a sample of the organic layer was saved for analysis as well.
• saturated brine 650 mL
• MTBE 15O mL
• the second aqueous layer was removed (800 mL) and saved for analysis. Samples of the phases were analyzed via 31 P NMR (monitoring for diethylphosphite content) and HPLC (monitoring the amount of 6-hydroxybuspirone within a given phase).
• a 4 L cylindrical, glass reactor equipped with mechanical stirrer, condenser, and thermocouples (for both batch and distillate temperature) was charged with 3800 mL of the solution. Distillation was conducted under reduced pressure (the pressure ranged from -19.5 to -20 in Hg) until the volume was approximately 500 mL. The remaining product-rich organic phase was added and distillation resumed, reducing the volume to approximately 1000 mL. IPA was added (2000 mL) and distillation at reduced pressure was resumed until the volume was reduced to approximately 1000 mL. An additional 100O mL of IPA was added bringing the volume to approximately 2000 mL (no THF was detectable via GC and the water content was measured to be 0.13%).
• Gepirone (4,4-dimethyl-l-[4-[4-(2-pyrimidinyl)-l-piperazinyl]butyl]-2,6- piperidinedione) (10.0 g, 27.8 mmol) was charged to a 500 mL flask equipped with a mechanical stirrer and a React-IR probe under inert gas. Tetrahydrofuran (250 mL, 25 mL/g) was charged and the mixture agitated at ambient temperature until homogeneous. Triethyl phosphite (28.9 g, 174 mmol, 29.8 mL, 6.25 eq) was added and the mixture was cooled to -65 to -80 °C.
• the mixture was agitated at this temperature for at least 10 minutes to allow the React-IR signal to stabilize.
• 1.0 M Sodium bis(trimethylsilyl)amide in THF (27.8 mL, 27.8 mmol, 1.00 eq) was charged to the mixture at such a rate so as to maintain the temperature less than -60 °C.
• Small amounts of sodium bis(trimethylsilyl)amide were charged to the mixture until the IR signal for buspirone reached a minimum indicating complete deprotonation of gepirone.
• Additional gepirone in THF 25 mlJg was then charged to the reaction 5 mixture in small increments until the IR signal indicated a 3.24 % excess of gepirone.
• Air was sparged into the reaction mixture, controlling the initial rate of sparging so as to maintain the temperature of the reaction mixture less than -60 °C. The sparging was continued until the reaction was complete as indicated by HPLC. Methyl tert-butyl ether (40.0 mL) was added followed by 1M hydrochloric acid (45.0 mL) O and the solution was warmed to ambient temperature. The pH (9.48 at 20.6 °C) was adjusted to between 6.5 and 6.9 using hydrochloric acid and Na 3 PO 4 (pH 6.95 at 22.7 °C). The phases were separated and the organic phase was washed twice with 25 wt % brine (40.0 mL).
• the solvent of the rich organic layer was then replaced by isopropyl alcohol and the solution was cooled to ambient temperature to crystallize 5 the reaction product.
• the crystalline slurry was then filtered and the wet cake was washed twice with isopropyl alcohol (15.0 mL) and dried to provide 3-hydroxygepirone (9.32 g, 89%), mp 128 °C.

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